A fuel cell is a device that directly converts chemical energy in the form of a fuel into electrical energy by way of an electrochemical reaction. In general, like a battery, a fuel cell includes a negative electrode or anode and a positive electrode or cathode separated by an electrolyte that serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively.
In order to produce a useful amount of power, individual fuel cells are typically arranged in stacked relationship in series with an electrically conductive separator plate between each cell. A fuel cell stack may be categorized as an internally manifolded stack or an externally manifolded stack. In an internally manifolded stack, gas passages for delivering fuel and oxidant are built into the fuel cell plates themselves. In an externally manifolded stack, the fuel cell plates are left open on their ends and gas is delivered by way of manifolds or pans sealed to the respective faces of the fuel cell stack. The manifolds provide sealed passages for delivering fuel and oxidant gases to the fuel cells and preventing those gases from leaking either to the environment or to the other manifolds. In some fuel cell stack arrangements, the stack is placed in an enclosure and the enclosure environment represents one of the process gases. In such a system, at least three manifolds are required to provide inlet and outlet gas passages for the stack, each of which must be sealed to the stack. In any case, the manifolds must perform the above functions under the conditions required for operation of the fuel cell stack and for the duration of its life.
An important aspect of the performance of an externally manifolded fuel cell stack is the flow distribution of process gases, i.e. fuel and oxidant process gases, among multiple fuel cells so that the process gases are evenly distributed to each of the fuel cells in the stack. In particular, it is desired that in a stack of fuel cells, all of the fuel cells in the stack get about the same amount of gas flow therethrough for the electrochemical reaction and for the purpose of cooling the cells uniformly or in predetermined regions. In conventional externally manifold fuel cell assemblies, fuel cells that are disposed closest to the anode exhaust pipe receive more fuel gas flow than fuel cells disposed further away from the exhaust pipe. As a result, the fuel cells with less gas flow therethrough are subjected to greater overheating while fuel cells with more gas flow produce more electrochemical reactions and thus, more power. This uneven gas distribution leads to a decreased fuel cell stack lifespan.
Therefore, there is a need for an externally manifolded fuel cell stack for a greater control of the flow distribution of the gases to the fuel cells in the stack to more uniformly distribute the gas flow to all of the fuel cells in the stack. However, controlling of the fuel gas supply to individual fuel cells within the stack requires a complex piping and fuel delivery assembly. In addition, there is a need for controlling the distribution of fuel so as to provide cooling to the anode inlet area of the stack.
It is, therefore, an object of the present invention to provide a manifold assembly that is capable of controlling the gas flow distribution through the fuel cells in the stack.
It is a further object of the invention to provide a manifold assembly which has a simple construction and may be adjusted for a desired flow distribution profile.
At a system level, the fuel cell system may include multiple fuel cell stacks that operate from a single fuel source provided by a fuel intake assembly and/or a single oxidant source provided by an oxidant gas intake assembly and/or exhaust spent gas into a single exhaust assembly. In such a multiple fuel cell stack system, each stack requires a predetermined amount of fuel and/or a predetermined percentage of the fuel in air. As flow control valves are costly and fragile in the hot, corrosive environments that surround the fuel cell stacks, it is desirable to have a flow control assembly that does not depend on such costly, fragile valves.
It is, therefore, a further object of the invention to provide a baffling assembly that has a simple construction and is capable of controlling gas distribution to fuel cell stacks in a multiple fuel cell stack system.